Are you a labour supporter wanting to protest vote? In the light of various politicos saying 'a vote for Lib Dem could let the Tories in by the back door', it could be a good idea to look at the stats.
Have a look at these statistics. Assuming that the lib dems improve their performance from 2001 and get a 22% share of the vote (from around 18%), then labour still get a majority even if the Tories lead by 7%!